The present invention is directed to new and useful improvements in centrifugal machines and in particular to a nozzle type centrifuge having vertically spaced pumping chambers and a bottom feed pipe for introducing a feed slurry upwardly into the centrifuge.
In one nozzle type centrifuge of the type mentioned, a double cone-shaped rotor bowl is provided having a separating chamber comprised of a stack of rotatable separating discs for effecting a two fraction separation of the bottom feed slurry into a heavy nozzle discharge for an underflow fraction at the periphery of the bowl and a light fraction discharge as an overflow at the top of the machine.
Centrifuges of this type are well known and are disclosed in prior art patents such as U.S. Pat. No. 4,005,817 which issued Feb. l, 1977 and U.S. Pat. No. 4,067,494 which issued Jan. 10 1978. In the centrifuge embodying the present invention such as disclosed in the latter patent a portion of the underflow fraction is returned to the separating discs from an annular return pumping chamber at the base of the rotor bowl. The feed slurry is introduced by injection from the bottom of the centrifuge upwardly into the rotor bowl via a second annular main pumping chamber which is spaced vertically from the return pumping chamber. These pumping chambers propel their respective slurries upwardly to the separating discs in the rotor bowl. The mentioned portion of the underflow which is recycled or returned back to the return pumping chamber enters the return impeller therein and it is pumped to the periphery of the rotor to be discharged again by discharge nozzles. In these centrifuges the rotor arrangement requires that the feed and return impellers be located respectively in spaced vertically adjacent pumping chambers arranged in superimposed relationship. Due to mechanical requirements a passageway is provided between the two chambers adjacent the main feed slurry pipe.
In use problems have been encountered in that a portion of the feed slurry can spill from the main pumping chamber through the passageway to the return pumping chamber. As a result the feed can enter the peripheral underflow discharge zone of the rotor bowl via the return chamber and dilute the underflow product. It is also possible that the diluted return can create buoyancy currents in the peripheral zone of the rotor. These factors can adversely affect the efficiency of the centrifuge in certain applications of use such as starch gluten thickening processes by upsetting the density gradients in the thickening zone of the centrifuge.
It is the object of the present invention to provide a bottom feed centrifuge having novel sealing means between the feed and return pumping chambers.
Another object is to provide a novel sealing means for reducing spillage between the feed and return pumping chambers at the passageway between the two chambers.
Another object is to provide a seal having means for adjusting the position of the seal to vary the gap between the return and feed pumping chambers.